1. Field of the Invention
Embodiments of the present invention generally relate to the manufacturing of optoelectronic devices, such as light emitting diodes (LEDs), laser diodes (LDs) and, more particularly, to processes for enhancing the light extraction by surface roughening of Group III-V materials used in optoelectronic devices.
2. Description of the Related Art
Group III-V materials are finding greater importance in the development and fabrication of a variety of semiconductor devices, such as short wavelength LEDs, LDs, and electronic devices including high power, high frequency, high temperature transistors and integrated circuits. For example, short wavelength (e.g., blue/green to ultraviolet) LEDs are fabricated using the Group III-nitride semiconducting material gallium nitride (GaN). It has been observed that short wavelength LEDs fabricated using GaN can provide significantly greater efficiencies and longer operating lifetimes than short wavelength LEDs fabricated using non-nitride semiconducting materials, comprising Group II-VI elements.
One method that has been used for depositing Group III-nitrides, such as GaN, is metal organic chemical vapor deposition (MOCVD). This chemical vapor deposition method is generally performed in a reactor having a temperature controlled environment to assure the stability of a first precursor gas which contains at least one element from Group III, such as gallium (Ga). A second precursor gas, such as ammonia (NH3), provides the nitrogen needed to form a Group III-nitride. The two precursor gases are injected into a processing zone within the reactor where they mix and move towards a heated substrate in the processing zone. A carrier gas may be used to assist in the transport of the precursor gases towards the substrate. The precursors react at the surface of the heated substrate to form a Group III-nitride layer, such as GaN, on the substrate surface. The quality of the layer depends in part upon deposition uniformity which, in turn, depends upon uniform flow and mixing of the precursors across the substrate.
While the feasibility of using GaN to create photoluminescence in the blue region of the spectrum has been known for decades, there were numerous barriers that impeded their practical fabrication. For example, the external quantum efficiency for GaN based LEDs is limited by their inability to emit all of the light that is generated by the active layers. When an LED is energized, light emitting in all directions from its active layer (e.g., InGaN multi-quantum-well (MQW) layer) reaches the emitting surfaces at many different angles. However, according to Snell's law (also known as the law of refraction), light traveling from a region having a higher index of refraction to a region with a low index of refraction that is within a certain critical angle (relative to the surface normal direction) will cross to the lower index region. Considering the refractive indexes of GaN (n>2.4) and air (n=1), the critical angle for the light to escape is about 23°. Light that reaches the surface beyond the critical angle will not cross but will be repeatedly reflected into the substrate and trapped inside the active layers, unless it escapes through the sidewalls. Assuming the light emitted from sidewalls and the backside is neglected, it is expected that approximately only 4% of the internal light can be extracted. Due to this phenomenon, the light extraction efficiency has not been satisfactory since the majority of lights generated by conventional LEDs do not emit as expected.
As the demand for LEDs, LDs, transistors, and integrated circuits increases, the efficiency of depositing high quality Group-III nitride layers takes on greater importance. Therefore, there is a need for an improved process and apparatus that can increase the light extraction efficiency while maintaining the consistent layer quality over larger substrates.